An excerpt from Trailer Training: An Easy guide to the Proven Methods of John Lyons:

It's important to keep our training simple, so it is imperative that you concentrate on moving a spot on the horse rather than moving "the horse" or even "the rear end." Remember, prey animals are really great at reading body language – it's how they survive. More than that, they are excellent at figuring out specifically where you're staring. (In the same way that you can tell when somebody's looking in your eyes.) We use these traits to our advantage as described here, keeping things simple and objective when we "narrow our focus."

Lastly, I want you to get the horse backing up based on your body language. Reread yesterday's lesson if you need to, but it comes down to this: Ask your horse to move forward. (You should be able to do this with zero pressure on the lead rope, without any tapping, just your stance.) Ask the horse to pivot on his front end (on that shoulder closest to you) and then move directly at the horse, asking him to back away. Kiss and thoroughly expect him to move. It really helps, by the way, if you walk toward the horse as if it doesn't exist, looking right through him – otherwise you'll begin to slow down, your horse will read that – and he'll just stand there. At this point I'll apply a tiny bit of downward pressure on the lead, suggesting that the horse keep his head down, level with his withers. I release the second I feel any relaxation. This just kind of helps instill in the horse "Hey, walk backwards without throwing your head up and hollowing your back."